The path of aerosols that reached California in 2011. Circled numbers indicate locations in which dust was captured in CALIPSO images. Image: Science.

A field study of aerosol impacts on clouds and precipitation in the Sierra Nevada shows that dust and microorganisms transported from as far away as the Sahara desert help to spur the precipitation that California counts on for its water supply.

The CalWater field campaign, funded by the California Energy Commission and led by UC San Diego and NOAA, could help western states better understand the future of their water supply and hydropower generation as climate change influences how much and how often dust travels around the world and alters precipitation far from its point of origin.

"UC San Diego is a leader in addressing complex, multi-disciplinary global challenges, such as water shortages and environmental concerns," said UCSD Chancellor Pradeep K. Khosla. "Our researchers work collaboratively to investigate and produce meaningful and impactful research that will further our understanding of our planet and environment, so we can improve human life and our world."

Jessie Creamean, a postdoctoral associate at NOAA's Earth System Research Laboratory in Boulder, Colo., co-authored the paper appearing in the journal Science with Kaitlyn Suski, a graduate student in the laboratory of Distinguished Chair in Atmospheric Chemistry Prof. Kimberly Prather, who holds appointments at Scripps Institution of Oceanography and the Department of Chemistry and Biochemistry at UCSD.

"We were able to show dust and biological aerosols that made it from as far as the Sahara were incorporated into the clouds to form ice, then influenced the formation of the precipitation in California," said Creamean, who conducted the fieldwork as a UCSD graduate student under Prather, the study leader.

"To our knowledge, no one has been able to directly determine the origin of the critical aerosols seeding mid-level clouds which ultimately produce periods with extensive precipitation typically in the form of snow at the ground."

The study, "Dust and Biological Aerosols from the Sahara and Asia Influence Precipitation in the Western US," appears Feb. 28 in online versions of Science.

Researchers have long known that winds can carry aerosols such as dust at altitudes above 5,000 meters (16,400 feet) from continent to continent. An unrelated 2009 study found that in one instance, Asian dust made a complete circuit around the planet in 13 days.

These dust particles can act as ice nuclei within clouds at warmer temperatures than would occur in their absence. They initiate the freezing of water vapor and water droplets, then precipitate as rain, snow, or hail depending on whether meteorological conditions enable them to attain sufficient mass to fall from the sky before evaporating. Without ice nuclei, ice would likely not form in clouds with temperatures above -38 degrees C (-36.4 degrees F).

Besides dust, aerosols can be composed of sea salt, bits of soot and other pollution, or biological material. Bacteria, viruses, pollen, and plants, of both terrestrial and marine origin, also add to the mix of aerosols making the transcontinental voyage.

The researchers' analysis of winter storms in 2011 found that dust and biological aerosols tend to enhance precipitation-forming processes in the Sierra Nevada. In previous studies, researchers have found that pollution particles have the opposite effect, suppressing precipitation in the Sierra Nevada.

The bulk of the data collected during CalWater came from instruments known as aerosol time-of-flight mass spectrometers (ATOFMS), co-developed by Prather, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, which tracked the transport of aerosols through the atmosphere from continent to continent.

Measurements in and around clouds utilized the Department of Energy's G-1 research aircraft, which carried other vital instruments, such as a specialized detector for the presence of dust ice nuclei feeding clouds and their presence in the collected residue of ice crystals. That portion of the study was led by co-author Paul DeMott, a senior research scientist at Colorado State University.

Using these tools, the researchers were able to determine that at least some of the dust and bioparticles detected by an aircraft-mounted ATOFMS unit during February 2011 flights through Sierra Nevada storm clouds were in the skies over Oman 10 days earlier, having likely originated in the Sahara a few days earlier.

Along the journey, the Saharan dust and microbes mixed with other aerosols from deserts in China and Mongolia before wafting over the Pacific Ocean. Upon arrival in California, the aerosols effectively seeded the storm clouds and contributed to the efficiency of clouds in producing precipitation. Two other transportable ATOFMS units housed in trailers at Sugar Pine Dam just south of Interstate 80 in the Tahoe National Forest and other instruments made further measurements. They determined the chemical composition of aerosols at the end of their journey by looking at the particles present in precipitation samples that were collected during storms.

The researchers said it is a major challenge to sort out the relative impacts of meteorology, atmospheric dynamics, and the original sources of the cloud seeds on precipitation processes. They added that further studies like CalWater are necessary to further identify which aerosols are conducive to precipitation formation and which aerosols stifle its production.

"Due to the ubiquity of dust and co-lofted biological particles such as
bacteria in the atmosphere, these findings have global significance," the study concludes. "Furthermore, the implications for future water resources become even more substantial when considering the possible increase in [wind-blown] dust as a result of a warming climate and land use changes."

"Hydropower is an essential source of electricity in California providing, on average, 15 percent of our annual generation. More importantly, it provides electricity during hot summer days when it is needed the most," said Energy Commission Chair Robert B. Weisenmiller. "This state-funded study in cooperation with NOAA will help us understand how small particles in the air affect precipitation and hydropower generation. Additionally, this information will be useful in estimating the effects of our changing climate."

Besides Creamean, Suski, and Prather, study coauthors include Daniel Rosenfeld of the Hebrew University of Jerusalem, Alberto Cazorla of UCSD, Paul DeMott of Colorado State University, Ryan Sullivan of Carnegie Mellon University, Allen White, F. Martin Ralph of NOAA, Patrick Minnis of NASA's Langley Research Center, and Jennifer Comstock and Jason Tomlinson of the Pacific Northwest National Laboratory in Richland, Wash.

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